With the availability of a practical extraterrestrial payload delivery system (e.g. the space shuttle), science and engineering research and development facilities have accelerated their efforts to design and construct orbital systems (e.g. space station and orbiting defense weaponry), where advantage can be taken of weightlessness and the absence of the electromagnetic absorption and reflection characteristics of the earth's atmosphere. Power for such systems is expected to be supplied by large, point-focusing solar energy devices which will use extensive (e.g. parabolic reflector) surfaces in order to concentrate the sun's energy and the attain the temperatures needed to operate efficient cycles for generating the vast quantities of electricity projected to be used.
Currently developed point-focussing devices that reflect sunlight into a solar energy receiver may be grouped or classified into three principal configurations: 1-large fixed dishes fed by an array of flat mirrors (heliostats) which track the sun; 2-fixed tower mounted receiver arrangements with sun tracking heliostats individually aimed at the focal point receiver; and 3-gimbal mounted symmetric parabolic dishes which track the sun directly (analogous to radio telescopes).
The fixed-dish approach is secured to an anchored, solid structure. Consequently, a very accurate paraboloidal reflector (composed of small, rectangular, second surface glass, with spherically contoured facets to allow accurate aiming toward the focal point) can be produced, making this type of solar energy collection system a candidate for terrestrial construction. On the other hand, because of its considerable size and mass, its attendant heliostats, and double reflected energy loss, it is not a practical solution to spaceborne applications.
The power tower approach eliminates the double reflectance loss of the fixed-dish, but requires an inordinate amount of space to accommodate all the heliostats (which must be aimed at the same point without mutual blockage).
The third type of device, the gimballed parabolic dish, provides a compact package with acceptable optical efficiency. However, unlike a radio telescope receiver horn, its solar energy receiver is massive, and imposes not only a shading loss due to the shadow it and its support structure casts on the concentrator, but a substantial weight penalty, as the mirror surface and receiver positions must not deflect significantly as the collector tracks the sun. If the power conversion and condenser units are located remote from the receiver, hot fluid lines must be run along the receiver support struts, giving rise to additional thermal losses and structural distortion. If the power conversion and condenser units are co-located with the receiver at the focal point, additional shading and support penalties are incurred.
One proposal that has been suggested for solving the problems of such configurations has been to use a Cassegrainian optical configuration. However, upon closer examination several reasons why such a concentrator has been not been successfully constructed become apparent. First, a Cassegrainian configuration incurs a double reflectance loss, and the secondary reflector produces at least as much shading as a receiver. The lower total reflectance necessitates a larger primary reflector to gather the same amount of energy as a basic Newtonian dish. Also, the secondary reflector can be made only so small before the concentrated sunlight from the primary either melts the secondary reflector or mandates the use of a secondary cooling system.
On the other hand, compatibility with presently available receiver configurations requires the use of a relatively large secondary reflector to produce the desired `cone angle` (typically 90.degree. to 120.degree.). A sufficiently large secondary reflector would have to be half the size of the primary which, in turn, would have to be twice as large as a comparable Newtonian reflector! For a spaceborne application, this size penalty would eliminate any drag advantage over a photovoltaic approach. In addition, the secondary reflector would have to be a deployable structure, since it would be too large to fit into the space shuttle in one piece, with a collateral increase in alignment criticality.
In addition to point-focussing devices, there have been proposed line-focussing configurations employing an offset parabolic trough, such as described in U.S. Pat. No. 4,296,737. An offset trough provides a simpler reflective surface (curved in only two dimensions) and is thereby easier to fabricate than a paraboloid. However, because the reflective surface is only two dimensionally curved, it focuses sunlight along only a linear strip region of the back interior wall of the receiver, so that the collection area is limited (a line vs. the entire surface area of the interior wall of a cylinder). Consequently, the theoretical maximum concentration (watts/sq. meter) is less and thus inhibits system thermal efficiency. Moreover, the trough approach shares the bulkiness problem of the fixed dish and power tower configurations and the opto-structural problems of the gimballed dish design.
Thus, it will be appreciated that although a variety of solar concentrator designs have been proposed (including those for use in space applications), most involve relatively large, bulky structures which are limited in potential for growth by their mass and the stowed volume of their collectors. The fixed-dish and power tower approaches will not work in a planetary orbit, since they will not accommodate 360 degrees of sun-tracking. A parabolic trough has a significantly limited concentration ability; also, conventional parabolic dishes possess the above-discussed shortcomings. Still, designers have continued to work on proposals that enable a symmetric dish concentrator to be practically space-deployable (in terms of stowability and deployment from its transport vehicle (space shuttle cargo bay). These proposals include petalline and hexagonal panel geometries, various inflatable designs and electrostatically controlled membranes.
Petalline geometries are radially symmetrical and can be scaled. However, they suffer from poor packaging efficiency and the need for very large spherical optics.
Hexagonal panel systems have significantly improved modularity and offer enhanced packaging efficiency and ease of deployment over petalline designs. However, in sizes that can be stowed efficiently within a transport vehicle (e.g. the space shuttle) the individual hex panels still require the fabrication of large, aspheric optical surfaces. (It should also be recalled that flat or substantially planar hexagonal panels that conform with one another edgewise to form a prescribed two dimensional shape do not readily `nest` together to form a three dimensionally contoured surface, as the dimensions become drastically distorted over the three dimensional surface.)
Inflatable configurations (balloons) stow more efficiently than hexagonal panel systems. However, sunlight must pass through the balloon to reach the reflective surface, thus constraining the choice of materials used. In addition, maintaining the accuracy of the surface geometry is debatable at best, and cannot be overcome by increasing the size of the reflector.
The electrostatically controlled membrane is not yet a practically deployable structure since it must first be radially pretensioned against a sturdy hoop structure and then provide a very stiff back-up for the million or so electrostatic actuators that would be required to control the surface contour. In addition, even if the hoop and back-up structures could be made lighter than an equivalent dish and the controls made reliable enough for long life, there is still some question as to the effect that slicing through the earth's magnetic field at orbital velocity would have on the delicate electrostatic forces shaping the surface.